Explosions rock Boston Marathon; several injured (video)

(CNN) — Two large explosions rocked an area near the finish line of the Boston Marathon on Monday, injuring at least six people and spewing wreckage in its wake.

Smoke billowed into the air at Copley Square, turning a site of celebration into a mess of destruction.

Crowds had gathered in the area to watch the runners.

At least four people were being treated at Massachusetts General Hospital.

It was not immediately clear what caused the blasts. Authorities rushed to assist the injured and close off the scene.

Out of an abundance of caution, the Lenox Hotel was evacuated, the Boston Globe reported, as authorities looked into possible security concerns.

Video from the moment of one of the blasts showed some people who appeared to be knocked over and then got up, but the video did not clearly show spectators who may have been closer to the site of the blast.

Mike Baingon, who works at the Atlantic Fish Company, said an explosion took place in front of the restaurant and that he was right by the front door at the time.

The explosions occurred at about 2:45 p.m., more than two hours after the first of the race’s nearly 27,000 runners had crossed the finish line, CNN Producer Matt Frucci reported.

The race was halted after the explosions as was subway service into the area.

Troops from the Massachusetts National Guard were assisting police as well.

In New York City, authorities stepped up security as well “until more about the explosion is learned,” Deputy Police Commissioner Paul Browne said.